Ryanodine does not affect calcium current in guinea pig ventricular myocytes in which Ca2+ is buffered

Calcium current in mammalian ventricular muscle is altered in the presence of ryanodine. Previous studies performed on rat ventricular cells have shown a slowing of Ca2+ current inactivation and suggest the hypothesis that ryanodine, by reducing the release of Ca2+ from the sarcoplasmic reticulum, reduces the availability of Ca2+ for inactivation of Ca2+ current (Ca(2+)-dependent inactivation). Another hypothesis is that the effects of ryanodine on Ca2+ current are due to a mechanical connection of the ryanodine receptor with the L-type Ca2+ channel. To further test these hypotheses we examined the effect of ryanodine on Ca2+ current in single voltage-clamped guinea pig ventricular myocytes that contained Ca2+ indicator and Ca2+ buffer. We used fura 2 (pentapotassium salt) to confirm that the ryanodine we used was capable of abolishing Ca2+ release from the sarcoplasmic reticulum during the period in which it was present. We perfused the cells with 10 mM EGTA to block changes in intracellular Ca2+ concentration. In the absence of internal EGTA, Ca2+ currents displayed biexponential inactivation and Ca(2+)-dependent inactivation (steady-state inactivation curves turned up at positive potentials). Inactivation was slowed by ryanodine at 10 microM. In cells perfused internally with EGTA, however, ryanodine had no effects, and steady-state inactivation curves were not shifted to the right. We conclude that, in guinea pig ventricular myocytes, the effects of ryanodine on Ca2+ current are mediated by Ca2+ and thus the effects of ryanodine do not provide a basis on which to postulate a physical connection between the L-type Ca2+ channel and the ryanodine receptor (sarcoplasmic reticulum Ca2+ release channel).